Optical fibre cables carrying optical fibre transmission lines have heretofore been installed by the same methods as conventional metal conductor cables. Such methods usually involve pulling the cable with a pulling rope through a previously laid cable duct. Frequently the cable duct already contains one or more conventional cables at the time of installing the optical fibre cable.
Unlike the metal conductors of a conventional cable, the optical fibres are easily damaged by tensile stress. Such stress may, for example, propagate micro-cracks, leading to fibre breakage in the long term. It is, therefore, standard practice to reinforce optical fibre cables by providing a central strength member, usually one or more steel tension wires, about which the optical fibres are disposed. The strength member takes up, and thus increases the ability of the cable to withstand, tensile stresses accompanying installation of the cable.
Unfortunately, the central strength member usually provides insufficient protection against local stresses caused by pulling a further cable through the same duct. The conventional approach of installing at the outset optical fibre cables containing sufficiently large numbers of optical fibres to satisfy foreseeable future traffic demands is a way of overcoming this problem. In consequence, first time installation of optical fibre cables containing dozens or even hundreds of optical fibres are currently envisaged despite the fact that to begin with a small fraction of the installed fibres would provide ample traffic carrying capacity. A further reason for installing optical fibre cables of comparatively large dimension is that the smaller the cross section of the cable the more prone the cable becomes to wedging in between those cables already present in the duct.
The first time installation of large diameter optical fibre cables with high numbers of optical fibres, is, however, undesirable for a variety of reasons. Firstly, there are problems of a technical nature inherent in such cables, such as, for example, the difficulty of forming joints and of achieving the required high strength-to-weight ratios. Secondly, there are clear economical drawbacks in committing large resources to install initially unused fibre capacity, particularly in view of the comparatively recent origins of optical fibre technology which lead one to expect continued substantial reductions in the price and improvement in the quality of optical fibres. Thirdly, there is the serious risk of damaging in a single incident very large numbers of expensive optical fibres. Finally, there is an appreciable loss in flexibility when routing high density optical fibre transmission lines.
A method of installing optical fibres with pulling ropes and pull chords is described in "Sub-ducts: The Answer to Honolulu's Growing Pains", Herman S L Hu and Ronald T. Miyahara, Telephony, 7 Apr. 1980, pp 23 to 35. The installation method described there proceeds as follows: a section of existing 4-inch (100 mm) duct is rodded and thereafter between one and three individual 1-inch (25 mm) polyethylene tubes are inserted into the duct using pulling ropes. The polyethylene tubes form subducts into which an optical fibre cable can be pulled with the aid of a nylon pull chord which has previously been inserted into the subduct by means of a parachute attached to its leading end and pushed through the subduct with compressed air.
The method just referred to does deal with some of the problems discussed above, but only to a very limited extend. Thus, it enables fibre capacity to be increased in up to three stages, and separates the optical fibre cables from those cables already in the duct, thereby greatly reducing the likelihood of jamming, and hence overstressing, of the optical fibre cable.